The head of the World Health Organization (WHO), Margaret Chan, has declared that the Zika virus is "spreading explosively" and has called for an emergency meeting of independent experts to decide if the outbreak should be declared an international health emergency.

The emergency meeting, scheduled for February 1, will examine the outbreak in Central and South America of the virus, which has been linked to birth defects and neurological problems.

Chan said she is "deeply concerned" about Zika because of the prospect of further spread by mosquitoes, a lack of immunity among people in newly affected areas, and the absence of vaccines, medications, or quick diagnostic tests for the virus.

Since October 2015, there have been nearly 3,900 confirmed cases in Brazil – which is due to host the Olympic Games in August.

WHO says Central and South America could see 3 to 4 million cases of Zika infection during the next year.